Powdery metallurgical products are now generally produced by press-molding a mixed powder of metal powder and a lubricant such as zinc stearate after packing the mixed powder into a die; and performing a defatting step and sintering step in an inert atmosphere or in a reducing atmosphere. In these cases, the shape of the product is retained by the mechanical tangling of the metal particles by the outer force exerted during the pressing in the die. The lubricant is added in an amount of about 0.5 to 1% by weight based on the metal powder, and mainly contributes to the promotion of the releasing property of the product and promotion of the packing property of the material powder into the die.
On the other hand, a process for producing a porous metal body is known wherein an organic porous body made of a resin foam such as polyurethane foam or the like is coated with a slurry containing metal powder and an organic binder, is defatted and sintered to obtain a porous metal body (see, for example, Patent Literature 1). By this method, before the initiation of the sintering of the metal powder, the shape is retained by the polyurethane foam at lower temperatures, and by the organic binder in the temperatures higher than the decomposition temperature of the polyurethane foam.
As the organic binder which is required to exist without being decomposed up to the sintering initiation temperature, a substance which is easy to be carbonized, such as a phenol resin, is used in many cases. With a metal which is easy to be reduced such as nickel or copper, the region wherein carbon is oxidatively decomposed and the metal, for example, nickel is reductively sintered is the Region I in the Ellingham diagram shown in FIG. 1. Since this Region I exists in the area higher than 500° C. which is relatively cold, and the widths of the oxidation-reduction conditions of carbon and the oxidation-reduction conditions of nickel are large, a porous metal body having decreased residual carbon amount and decreased residual oxygen amount can be produced by controlling the composition of the atmosphere during sintering.
Patent Literature 1: JP 6-158116 A